Transmission of moving pictures in real-time is employed in several applications like e.g. video conferencing, net meetings and video telephony.
Video conferencing systems allow for simultaneous exchange of audio, video and data information among multiple conferencing sites. Systems known as Multipoint Control Units (MCUs) perform switching functions to allow the endpoints of multiple sites to intercommunicate in a conference. The MCU links the sites together by receiving frames of conference signals from the sites, processing the received signals, and retransmitting the processed signals to appropriate sites. The conference signals include audio, video, data and control information. In a switched conference, the video signal from one of the conference sites, typically that of the loudest speaker, is broadcast to each of the participants. In a continuous presence conference, video signals from two or more sites are spatially mixed to form a composite video signal for viewing by conference participants. When the different video streams have been mixed together into one single video stream, the composed video stream is transmitted to the different parties of the video conference, where each transmitted video stream preferably follows a set scheme indicating who will receive what video stream. In general, the different users prefer to receive different video streams. The continuous presence or composite image is a combined picture that may include live video streams, still images, menus or other visual images from participants in the conference.
As indicated above a number of endpoints using a multi-stream approach each may be sending its audio to all other endpoints and, if it is nominated as a “loudest speaker”, also sends its video. The video of the loudest speaker is either viewed alone, or as the largest view on the screen.
Voice switched is typically used to describe a conference where only one speaker is visible at any time. Continuous Presence refers to those where many speakers are visible, sometimes with the active speaker emphasised. Active presence is always the latter case
A similar situation would occur in a distributed MCU with multiple virtual endpoints mixing audio and video which is transmitted over the back plane. The MCU is in this case distributed across multiple physical chassis. The software architecture is arranged such that instead of using a back plane to communicate between the components, the Ethernet network whether LAN or WAN, is used. In the following description, endpoints are referred to both real endpoints and virtual endpoints. A virtual endpoint may be a unit representing a real endpoint in an infrastructure component like an MCU. The MCU consider the virtual endpoint to be the real endpoint, and exchanges all data to the virtual endpoint instead of the real endpoint. The virtual endpoint possibly formats and forward the data to the real endpoint. Data transmitted from the real endpoint to the MCU would be similarly treated via the virtual endpoint.
In certain scenarios, as a new speaker starts, an endpoint may switch very rapidly so as the first syllable is lost.